Efficient power conversion from a voltage source such as a battery or rectified and filtered AC source provides current for single or multiple loads is important in many consumer products. Load devices that require controlled current sources for proper operation include light emitting diodes (LEDs), LASERs, CPUs, battery chargers, lamps, and heater elements to name a few.
In lighting applications multiple matching current sources are required to reduce voltage stresses on power conversion components while insuring uniform brightness. Current sources must operate at high voltage levels to drive loads such as LEDs that are connected in series. For example 21 LEDs in series would require approximately 75V output capability to provide 20 mA of current to the 21 LED devices in series. If the same 21 LEDs are driven by 3 matching current sources each driving 7 LEDs than the output of each current source will only have to operate at 25V to provide 20 mA of LED current to the 7 LED devices in series. Often power converters are implemented as integrated circuits (ICs) where the voltage capability of the process used to make the integrated circuit (IC) has a negative impact on IC die area, which reduces yield and increases the cost of each IC.
Often a single power source is used to provide power that is used in different physical locations in an application. To reduce cost, loads that require a fixed current level must be supplied current from a single wire with the chassis providing a return path for the current. The power converter frequently has to step up the input voltage, which increases the current level in the wiring from the voltage source to the converter, and reduces the current needed to deliver power to the loads. Therefore, it is important to be able to locate the converter near the voltage source, and minimize the number of wires that are required to drive remotely located loads.
High efficiency power conversion is also important especially in applications where the input power source is a battery. Efficient operation increases the operating time between battery charges, and improves product reliability. Reliability is improved by decreasing the component power losses, and thus the heat that those losses produce.
Finally, portable products have size, weight, and cost limitations that are essential to meet the needs of present and emerging applications. Power conversion for generating current sources must be designed with high levels of integration and minimal component count.
Existing current source power converters typically use a single voltage to voltage converter such as a boost converter to produce a fixed voltage source. The fixed voltage source is used to provide current to the load or loads with a separate current sense element, series pass element, and control loop for each load. The current sense element, series pass element, and control loop regulate the load currents as the forward voltage drop of the loads vary due to temperature, life, or variations in the component manufacturing process.
Another approach to current source power converters is to use separate boost converters where each load requires a current sense element, series pass element, and control loop to maintain accurate current levels. This improves efficiency by allowing each stack of series load elements to operate at their minimum or most efficient voltage level. For example, if the forward voltage drop of the loads in series is 25V then the converter can be set to operate with a regulated voltage of 26V, where there is 1V across the series pass element used to regulate current. There is still a loss in the pass element used to control the load current, but it has been reduced. In each approach the load current is sensed continuously.
The single voltage to voltage converter approach with multiple loads, each having separate current sensing and current control loops has many disadvantages. As multiple channels are added to address multiple loads, multiple current sense elements and wires need to be added to provide feedback signals for the current control loops. The single voltage to voltage converter has to be set to a fixed voltage that is sufficiently high to supply current to the load with the highest voltage requirements. Loads that need less voltage have to dissipate power in the pass element to maintain current regulation. The approach requires precisely matched loads. Unmatched loads result in low efficiency. The current sensing components and current sense feedback wiring add cost, size and weight.
The use of multiple boost converters for each load with remote current sensing for feedback to a current control loop, and series pass element improves efficiency by allowing the voltage to be set to the minimum level that is required for current regulation. However, this approach adds an inductor and filter capacitor for each current source in addition to the wires and current sense elements that are needed to control current.
When the load current is sensed continuously, the current sense element can be selected to provide a high gain current sense signal. This improves the signal to noise ratio, but the sense element has to be a higher impedance element, which results in high current sense element power losses. The current sense element can be selected to lower sense element power losses, which improves efficiency of operation, but the current sense signal gain is decreased resulting in a lower signal to noise ratio.
There is a need for a voltage to current converter that can provide multiple current sources at differing voltage levels using a single inductor, without the need to provide a current sense element, pass element, and wiring to provide feedback for load current regulation. The existing solutions for driving current source loads do not meet the present or future needs of power conversion for current dependent loads. A new approach is needed.
It is therefore an object of the invention to provide a voltage to current power converter.
It is another object of the invention to provide a voltage to current power converter that is capable of supplying current to a plurality of loads using a single inductor.
It is another object of the invention to provide a voltage to current power converter that has regulated current source outputs without requiring current sense elements, pass elements, and wiring to provide feedback for load current regulation.
It is another object of the invention to provide a voltage to current power converter that operates more efficiently by not requiring continuous load current sensing.
It is another object of the invention to provide a voltage to current power converter with reduced weight, size, and cost by reducing the number of components that are required to supply current to a plurality of loads.